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Topic 5.4 Instrumentation Systems
Learning Objectives:
At the end of this topic you will be able to;
describe the use of the following analogue sensors:
thermistors and strain gauges;
describe the use of the following digital sensors:slotted discs (for sensing rotational speed),
encoded discs (for sensing angular position);
recall the Gray code (3 bit) and explain its use in encoded discs;
design and analyse sensor sub-systems which incorporate thermistors
and strain gauges in bridge circuits;
recall the adantages of a bridge circuit compared to a simple oltage
diider circuit;
recall, and explain the significance of, the ideal properties of an
instrumentation amplifier ! high input impedance and high common-
mode re"ection ratio;
analyse and design instrumentation amplifiers based on the op-amp
difference amplifier circuit;
select and use the formula:
#$%&' #**(+* +);
design a logic system to process the output of slotted and encodeddiscs to meet a gien specification.
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Module T5
lectronic Systems !pplications.
!nalogue sensing units:
An analogue signal can hae any oltage alue, limited only, usually, by theoltages of the power rails.
/ome deices hae a resistance which responds to changes in their
surroundings. *or example, a 0+ (0ight ependent +esistor) has a resistance
which decreases when more light falls on it. &here are two 1inds of
thermistor (temperature dependent resistor). &he ptc (positie temperature
coefficient) thermistor has a resistance that increases as its temperature
rises. &he ntc (negatie temperature coefficient) thermistor has a resistancethat decreases as its temperature rises. &his course considers only ntc
thermistors.
&he simplest form of sensing unit is made by connecting
one of these deices in series with a resistor.
&he output signal is ta1en from the point where the
resistor is connected to the deice.
&he diagram shows this arrangement used in a
temperature sensing unit. &he analogue output signal
changes as the temperature changes.
*or example, suppose that:
the thermistor has a resistance of 1. at a temperature of 24;
the ariable resistor is set to a resistance of 21;
the supply oltage #/' 2#.
&he output oltage is obtained from the oltage diider formula:
#$%&' #/(+ +5 +2)
n this case, at 24,
#$%&' 2 x (2 2 5 ) ' 6#.
"ercise # (&he solutions are gien at the end of the topic.)
At
4, a thermistor has a resistance of .71. &he ariable resistor isunchanged. 4alculate the output oltage of this temperature sensing unit at
64.
2
#
$ u t p u t
3 #
/
& h e r m i s t o r
# a r i a b l er e s i s t o r
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Topic 5.4 Instrumentation Systems
! strain gauge
8hen truc1s drie oer a bridge, or someone stands on bathroom scales, the
structure is s9uashed slightly. /train is defined as this change in length
diided by the original length, and strain gauges are used to measure it.
&he layout of a typical strain gauge is shown in the
picture. t is often glued to the structure, and so
is distorted when the structure is distorted.
&his changes the resistance of the strain gauge.
*or example, when a straight wire is stretched, it gets longer and thinner. Asa result, its resistance increases.
y measuring the change in resistance, we can monitor the strain thatproduced it.
&he strain gauge could be incorporated into a oltage
diider circuit, shown opposite, which behaes li1e the
one "ust considered for a temperature sensor.
oweer, there are problems with this 1ind of sensor:
. &he resistance of the sensor may change because of some factor otherthan the one you are trying to measure. *or example, the resistance of a
strain gauge changes if the strain gauge gets hot. &his has nothing to do
with any forces applied to it.
2. 0oo1 at the oltage diider formula again:
#$%&' #/(+ +5 +2)
As this shows, the output oltage depends on the supply oltage.
n many situations, the supply oltage will fluctuate.
&he system may be battery-powered and using batteries that aregoing flat.
3
#
$ u t p u t
3 #
/
/ t r a i ng a u g e
# a r i a b l er e s i s t o r
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&he system may be using a mains power supply, which does not
hae good line regulation.
&he power supply cables might be sub"ect to electrical noise,which changes the instantaneous alue of the supply oltage.
n other words, you cannot 1now whether a change in the output indicates
a change in the factor you are trying to monitor, or is the result of a
change in some other factor in its surroundings.
! better sensing circuit:
oth of the problems outlined aboe can be oercome or reduced by using abridge circuit, (though this title does not refer to the bridges that truc1s
drie oer. $ne is sub"ected to
the temperature changes under inestigation. &he other is not. &hat is the
only difference. $therwise, both thermistors are exposed to the same
conditions.
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Topic 5.4 Instrumentation Systems
$ull measurement tec%ni&ue:
%sually, the bridge is ?balanced@ initially. n other words, the ariable resistoris ad"usted until #$%&is ero.
n this condition:
+esistance of = ' +esistance of >
+esistance of ariable resistor +esistance of +
Bxercise 2 will show that in this condition, the power supply oltage ma1es no
difference at all. Any alue can be used, but two conflicting issues need to be
considered:
the higher the supply oltage, the more sensitie the output oltage is
to changes in temperature (for the temperature sensing bridge circuit.)
the higher the supply oltage, the greater the self-heating effect of all
the resistors in the circuit.
Any changes in the condition being monitored, e.g. temperature, ma1es the
bridge unbalanced, meaning that #$%&is no longer ero. %sing this null
measurement ma1es it possible to detect ery small changes in conditions, by
connecting the bridge circuit to a high gain oltage amplifier, as outlined
below.
! strain gauge bridge circuit:
As in the temperature sensing bridge circuit, there are two sensing deices.
n this case, they are labelled ?/train gauge@ and ?ummy strain gauge@, thoughtheir positions in the bridge can be reersed.
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lectronic Systems !pplications.
&he ?/train gauge@ is glued to the structure in such a way that it is distorted
by moement of the structure. &he ?ummy strain gauge@ is glued nearby so
that it is exposed to the same conditions, except for the distortion.
$ften, the two strain gauges are
formed on the same substrate,
as shown in the diagram.
"ercise ' (&he solutions are gien at the end of the topic.)
A thermistor bridge circuit is shown opposite.
&he power supply oltage #/' 2#
&he ariable resistor is ad"usted until the bridge
is balanced, i.e. the output #$%&' #. t then has
a resistance of exactly 2.71.
&hermistor > is found to hae a resistance
of .21.
(a)4alculate the resistance of thermistor =.
(b)&he power supply oltage is changed to #.
4alculate the new output oltage #$%&.
"ercise ( (&he solutions are gien at the end of the topic.)
nitially, the strain gauge bridge circuit is
balanced by ad"usting the ariable resistor,
which then has a resistance +' C.
0ater, a force is applied to strain gauge /,
and its resistance becomes 3DD.
&he dummy strain gauge, , is unaffected,
and still has a resistance of 37.
+2is a fixed resistor, with resistance C.
%se the oltage diider rule twice to calculate
the oltages at A and when the force is applied, and hence calculate the
output oltage #$%&.(All your calculations should be 9uoted to two decimal places.)
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Topic 5.4 Instrumentation Systems
Instrumentation ampli)iers:
&he output from a bridge circuit is usually only a few milliolts. t is usuallyamplified by a high gain oltage amplifier 1nown as an instrumentation
amplifier.
deal characteristics:
igh input impedance ! &his ensures that as much as possible of the
oltage signal from the bridge circuit is transferred to the
instrumentation amplifier. &he current flowing in the wires connecting
the two sub-systems is ery small, and so the oltage dropped acrossthe output impedance of the bridge circuit, (and so not transferred to
the amplifier) is 1ept to a minimum.
igh common-mode re"ection ratio !on both inputs. &here will be steady
4 oltages on the outputs of the two oltage diiders that ma1e up the
bridge circuit. (Eou calculated these earlier.) =art of this 4 oltage will
appear on, ( be commonto) both outputs. A high 4F++ ensures that the
instrumentation amplifier ignores (re"ects) these, and amplifies only thedifference between these oltages signals.
&he diagram shows the circuit for an
instrumentation amplifier, used in the
8B4 specification. t is based on an
op-amp difference amplifier. 8ith
careful design, it can match the ideal
characteristics ery well. t consists of
two pairs of resistors, labelled +and +*
and an op-amp. n practice, the circuits
for instrumentation amplifiers are more complex.
8hen used to amplify the output of a bridge circuit, input is connected to
output A, and input 2 to output of the two oltage diiders that ma1e up
the bridge circuit.
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lectronic Systems !pplications.
&he next section is H$& examinable< t shows you how to analyse the difference amplifier
circuit, using the rules for op-amps deeloped in earlier modules. f you can follow this, it
will help you to understand the way the circuit wor1s.
*+ules, )or op-amp be%aviour:
f the output is not saturated, the two inputs sit at the same oltage;
&he input impedance of the inputs is so big, they draw negligible current;
&he circuit shown will be used for
this analysis. &he sies of the
input oltages are much greater
than we would expect from abridge circuit, but will ma1e the
arithmetic easier